Patent GB-1232483-A (GB)
IPC: G01F23/26
Filed: 1968-09-17
Abstract: 1,232,483. Measuring capacitance. ROBERTSHAW CONTROLS CO. Sept.17, 1968 [Sept. 18, 1967], No.44180/68. Heading G1U. [Also in Division H3] A capacitance to current converter incorporates a constant current generator for supplying a constant primary charging current to a test capacitor Cx, a discharge circuit for discharging the capacitor repetitively when the voltage across it reaches a given value and generating an output signal proportional to the charge-discharge frequency, this signal controlling an output current generator. A feedback circuit responding to the output current generator provides a supplementary charging current to the capacitor, which may be adjusted to adjust the span of the converter. The constant current generator has a starting value adjustment such that the supplementary current will be zero for a given starting value of the test capacitor, the span and starting value adjustments being independent. The constant current generator incorporates a differential amplifier 10, Fig. 1, with reference voltage input E z  and current feedback controlled by R z , adjustment of R z  controlling the starting value. The given value to which Cx charges is determined by Eg , to voltage across Cx being compared with E R  in differential amplifier 16, whose output actuates a reset switch 14 to discharge Cx. The pulse output from amplifier 16 is passed via a shaper 18 and integrator 20 to one input of a further differential amplifier 22, where the integrated output e o  representing the charge-discharge frequency of Cx, is compared with a reference voltage E p . While the voltage is less than E p  , the amplifier 22 delivers an output which is used to control a current generator 24, providing the current output from the converter, to load terminals 26. A feedback voltage proportioned to this current is also developed across resistor R f  in the circuit of a transconductance differential amplifier 12, forming part of a supplementary current generator. The feedback voltage is compared with a reference voltage E n , and the supplementary generator develops an output current which is added to the output of the constant current generator so that the charging period of Cx remains constant, irrespective of its value. Initial adjustment is controlled by E n  with a suitable starting capacitance Cx in circuit until no voltage appears across R s  . The system zero may then be controlled by Rz and the span by R s  independently. In the detailed circuit, Fig.2 (not shown) a supply is obtained by rectifying an input A. C . (at T1, CR5) and regulating (at Q18). A transconductance differential amplifier (10) serving as the constant current generator, is provided by transistors (Q12, Q13, and Q11). The constant voltage source E z  is derived across a Zener diode (CR 3 ), and potentiometers (P 3 , P 4 ) serve to adjust the current feedback of the amplifier i.e. the zero. Constant current is fed via transistor (98) and a capacitor (C 7 ) to the unknown capacitance Cx, in parallel with a capacitor (C 6 ). When a given voltage, set by a Zener diode (CR 2 ) is reached, a transistor (Q9) conducts, triggering on a latch (Q10) which causes Cx to discharge. Output pulses from Q9 are passed through a reshaper (Q14-Q17) to an integrator (R 1 C 1 ). The output from this is compared with a reference E p  set by resistors (R 3 , R 41 ) by a differential amplifier (Q1) whose output controls the current delivered by the supply through a transistor (Q4) to output terminals (26). The voltage across a resistor (R f ) in series with Q4 controls one side of a transconductance differential amplifier (Q5, Q6) the other side being controlled by a voltage set by a potentiometer (P5) this voltage being applied via span adjusting potentiometers (P1, P2). The output from this circuit is added to that from the constant current generator via a transistor (Q8) and applied to the test capacitor.